As described in Non-Patent Literature 1, the features of micro-nano bubble typically include:                (a) bubble system is small,        (b) bubble rising speed is low,        (c) bubbles reduce frictional resistance,        (d) internal pressure of bubble is high,        (e) gas-liquid interface is large,        (f) amount of gas-dissolution is large,        (g) dissolution or shrinkage accompanies bubbles, and        (h) surface of bubble is negatively charged.        
Because of these various features, application of micro-nano bubbles to a wide range of fields such as foods, cosmetics, pharmaceuticals, semiconductor cleaning, and plant growth are expected. As the particle size becomes small, the buoyancy of micro-nano bubbles become very smaller compared to the viscous force. They can exist in a liquid remaining still in a form of ultra-fine bubbles for a long period of time without coming to the surface. Further, liquids containing nano bubbles of very small size are known to become transparent since such tiny bubbles are invisible.
Methods of generating micro-nano bubbles are classified roughly into two. One is to allow gas to accompany liquid by being fluidized in the liquid; the other is to blow gas into liquid in a stationary state. More specifically, as described in Non-Patent Literature 1, various micro-nano bubble generation methods have been proposed, the methods include a liquid swirling flow type, a static mixer type, a ventury type, a pressurized dissolution type, and a fine-pore type.
As stated above, the micro-nano bubble has such features as were not known in the conventional knowledge. In recent years, it has been studied to apply the micro-nano bubble to a cleaning method for peeling off or removing contaminants on substrates such as glass substrates or semiconductor wafers, wherein the contaminants include residual resist films and metals or metal compounds adhered or formed on the substrates (See Patent Literature 1, for example.) In the invention described in Patent Literature 1, at a first peeling-off process, a first stripping solution L1 having nano bubbles mixed therein is heated at a low temperature of about 40 to 60° C. and is supplied to the substrate at a condition that nano bubbles in the solution do not collapse so as to maintain high permeability to the resist film; then, at a second peeling-off process, the resist film is peeled off by the collapsing of the nano bubbles contained in the first stripping solution L1 using the pressure of a second stripping solution L2 given with a high-pressure pump.
Further, as an alternative to a lift-off method or an etching process of a resist film to be performed in order to peel off the metal film on the resist when manufacturing semiconductor devices, a method of jetting pure water containing ozone micro bubbles or micro-nano bubbles against a substrate has been proposed (see Patent Literatures 2 and 3.) Besides these, the inventors of the present invention have developed a new method of generating micro-nano bubbles in large quantities and an apparatus therefor. The developed new method and the apparatus are capable of generating micro-nano bubbles efficiently in quantity by the water hammering that is produced by pressurizing a gas-liquid mixture by high-pressure sending. In Patent Literature 4, the inventors of the present invention have proposed an application of this technique to a clean washing for such as semiconductor wafers.